1. Field of the Invention
The present invention generally relates to a sheet feeder and an image forming apparatus such as a copier, a printer, a facsimile machine, or a multifunction machine capable of at least two of these functions.
2. Description of the Background Art
Generally, there are two types of image forming apparatuses such as printers, facsimile machines, copies, or multifunction machines including at least two of these functions: ink-ejecting image forming apparatus using a recording head or liquid ejection head that ejects ink droplets onto sheets of recording media, and electrophotographic image forming apparatuses that develop electrostatic latent images with developer.
Ink-ejecting image forming apparatuses form images by ejecting ink droplets onto the sheet with the recording head while transporting the sheet. In such ink-ejecting image forming apparatuses, it is important to keep the ink in the recording head at a given negative pressure, that is, to keep the pressure exerted on the ink in the recording head at a given negative pressure, in order to eject ink reliably from the recording head. Therefore, ink-ejecting image forming apparatuses typically include a negative pressure generation unit disposed in an ink supply system for supplying ink to the recording head under negative pressure.
By contrast, electrophotographic image forming apparatuses form images by performing a series of interrelated processes, from electrical charging to image exposure, development, and transfer, on a photoconductor serving as a latent image carrier. More specifically, electrostatic latent images are formed on the latent image carrier and developed by a development unit into toner images. Then, the toner images are transferred onto sheets of recording media.
In both types of apparatuses, output images are formed on sheets fed to an image forming position or an image-transfer position from a sheet cassette or a manual feed tray provided separately from the sheet cassette.
Sheet cassettes generally include a feeding mechanism for picking up and conveying multiple sheets stacked therein one sheet at a time. For example, the feeding mechanism includes a feeding roller for conveying the sheet and a stack plate provided inside the sheet cassette that guides the leading edge portion of the sheet toward a feeding roller. The sheet is picked up and conveyed by the rotational force of the feeding roller when the sheet on the top of the multiple sheets stacked on the stack plate contacts the feeding roller.
Herein, it is possible that the sheets stacked on the stack plate curl. Sheets tend to curl when the rate of expansion and shrinkage of sheet fibers becomes uneven in the sheet, due to changes in temperature or humidity or after a first side of the sheet is heated in an image-fixing process in duplex printing. In addition to curling in a sheet width direction perpendicular to a longitudinal direction of the sheets, sheets can curl in the longitudinal direction when placed with the direction of fibers called a Y grain or long grain, which parallels the longitudinal direction of the sheet, in a direction in which the sheet is transported (hereinafter “sheet conveyance direction or sheet feeding direction”). In such a case, the sheet curls upward or downward in the sheet conveyance direction.
Regarding sheet feeding methods, for example, JP-3521039-B proposes a method in which the sheet cassette is provided horizontally and the stack plate inside the sheet cassette is lifted to bring the sheet into contact with the feeding roller. Alternatively, JP-3731849-B proposes a method in which the sheet cassette is angled with respect to the body of the image forming apparatus and the sheets stacked on the stack plate are forwarded to the feeding roller under the force of gravity (hereinafter “oblique feeding method”).
In either method, in order to separate a single sheet from the multiple sheets stacked on the stack plate, a frictional pad is provided obliquely to guide the leading edge portion of the sheet upward. In such a configuration, when multiple sheets are forwarded to the feeding roller at the same time, only the sheet on the top, which is in contact with the feeding roller, can be allowed to be separated from the other sheets by varying the frictional force between the multiple sheets, on the one hand, and the frictional force between the frictional pad and the sheet in contact with the frictional pad on the other. However, if the sheet curls, the sheet cannot be forwarded properly to the position where the frictional pad separates the sheet on the top from the multiple sheets.
Therefore, the above-described first approach proposes providing curved regulation members on the tops of side fences of the cassette to correct the curl of the sheet in the sheet width direction, and the above-described second approach proposes providing a roller or a rib that can press the upper surface of the sheet. The first approach is for correcting curl in the sheet width direction with both end portions in the sheet width direction curling upward, and the second approach is for correcting curl in the sheet conveyance direction with both end portions and a center portion in the sheet conveyance direction curling downward and bulging, respectively.
However, the leading edge portion of the sheet in the sheet conveyance direction can curl upward as well as downward. In such a case, the leading edge portion of the sheet is likely to contact the circumferential surface of the feeding roller. If such upward curl occurs in the configuration described above in which the leading edge portion of the sheet is lifted and is fed to the separation position by the feeding roller clamping the sheet together with an elevation member lifting the sheet, it is possible that the sheet cannot be fed properly, resulting in feeding failure or separation failure.
The above-described first and second approaches do not address feeding of sheets having upward curls in the sheet conveyance direction.
Sheets without upward curls can be guided relatively easily to the position where the sheet is sandwiched (hereinafter “sandwiched position”) because the direction in which the sheet is transported toward the sandwiched position is identical or similar to the direction in which the sheet passes the sandwiched position. However, for example, in a configuration in which the feeding roller rotates counterclockwise, and the clamped position is on a lower side of the circumference of the feeding roller, if the leading edge portion of the sheet in the sheet conveyance direction curls upward, which is opposite the direction in which the feeding roller rotates, it is difficult to guide the sheet to the clamped position using rotation of the feeding roller alone.
Such a problem may be solved by preliminarily causing the leading edge portion of the top sheet on the stack plate to contact the feeding roller in order to press the curling edge portion of the sheet with the circumferential surface of the feeding roller. However, in this method, for example, it is necessary to keep the leading edge portion of the stack plate lifted because the leading edge portion of the sheet must be kept lifted to be in contact with the feeding roller. As a result, the quantity of sheets stacked on the stack plate is reduced compared with a configuration in which the stack plate can be lowered, and accordingly frequency of replenishment of sheets increases.
In view of the foregoing, the inventor of the present invention recognizes that there is a need for a sheet feeder capable of feeding curling sheets properly and an image forming apparatus including the sheet feeder, which known approaches fail to do.